There’s a phenomenon in TTRPGs called a Mermaids Amulet. There was an item in a game that let a mermaid breathe in air, which was the ONLY thing that indicated they normally couldn’t. In short, a rule was only shown to exist by an ability to overcome it.

Monks have the ability to make a bonus action unarmed strike after making an attack, which would be redundant if the dual wielding rules let you do that.